1. Field of the Invention
The present invention relates to devices that deliver neural stimulation to the body, such as cochlear implants.
2. Related Art
Cochlear implant hearing prostheses are a well-recognized therapeutic means of restoring a sense of hearing for profoundly deaf persons. Using one or more microphones to convert incoming sound into electrical signals, these prostheses process the amplitude and frequency information contained within these signals to create several discrete channels of electro-neural stimulation for delivery to the human organ of hearing called the cochlear.
Since the hearing cells targeted by these prostheses are spatially distributed along the spiral pathways of the human cochlea, electrical stimulation intended to evoke perceptions of both the amplitude and frequency of incoming sound must be distributed spatially in a likewise fashion. As a consequence, each stimulation channel is thus assigned to deliver electrical stimulation in response to a particular range of incoming sound frequencies.
While thousands to perhaps millions of frequency distinguishing neural cells are implicated in normal hearing, current art electro-stimulation hearing prostheses are constrained by size and other factors to deliver only twenty or so discrete channels of stimulation. This number is sufficient to allow for speech recognition and conversation in quiet environments, but makes it difficult for speech to be easily discernible when background noise levels increase.
Users of languages predominantly more tonal than English are especially challenged since the limited number discrete channels of stimulation cannot replicate the inflection of a word to distinguish its meaning. It is also an unfortunate fact that while many users of current art devices report some enjoyment of musical sounds, tonal appreciation of even the simplest of tunes is beyond them as there are simply not enough stimulation channels available to fully restore the sense of hearing.
This limitation arises from the need to deliver a time constrained, minimum level of electric charge in order to evoke a hearing-like sensation with a particular apparent loudness or intensity. Current art electrodes are already close to the minimum size necessary to prevent non-reversible galvanic reactions, electrode corrosion, and the creation of chemical species that can destroy the viability of the sensory cells targeted by the delivered stimulation. These reactions increasingly occur as the electrode to body tissue interface impedance, and hence the voltage developed during the application of stimulation current, rises as the surface area of the electrode is reduced.
Since the avoidance of such reactions is crucial to the long-term success of these prostheses, current art intra-cochlear electrodes are required to be of a particular size so as to have an outer area of at least approximately 1 square millimeter. As a consequence, in addition to limiting the total number of stimulating electrodes that can be accommodated within the cochlear, the size of the electrode also limits the depth to which the intra-cochlear electrode array can be inserted within the cochlear and the degree to which low frequency hearing can be restored, since this where the neural cells associated with low frequency perception are located.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.